Adhesives comprising tackifying polymers as main components are called pressure-sensitive adhesives (tackifiers), and can be firmly adhered to materials by simply applying a pressure to the adhesives. When the adhesive contains a tackifying polymer and a crystalline polymer, it can be used as a heat-sensitive adhesive.
As such an adhesive containing a tackifying polymer and a crystalline polymer, for example, U.S. Pat. No. 5,192,612 (corresponding to JP-B-3021646) discloses a pressure-sensitive composition comprising a pressure-sensitive adhesive base resin (a tackifying polymer such as an acrylic polymer), a detackifying resin and a detackifying particulate. A specific preferable example of the detackifying resin is substantially linear polycaprolactone having a molecular weight of about 3,000 to about 342,000. Polycaprolactone is a crystalline polymer which exhibits non-tackifying properties at room temperature (about 15 to 30° C.).
The above-described pressure-sensitive adhesive can be adhered to an adherent by pressing the adhesive against the adherent. The above detackifying resin and the detackifying particles effectively reduce tack on the surface of the adhesive at room temperature and improve repeelability. The detackifying particles are inorganic particles such as silica.
In the above U.S. Patent, it is not supposed that the adhesive material is peeled (including thermal peeling) during or after the use of the adhesive material after the final adhesion. Therefore, the adhesive as such cannot be used as an adhesive having thermal-peeling-easy properties, which are described below. To impart the thermal-peeling-easy properties to the adhesive, the compatibility of the crystalline polymer and the tackifying polymer should be improved, as described below. However, the above U.S. Patent does not disclose the improvement of the molecular structure (the kinds of functional groups and/or substituents in the molecule, a weight average molecular weight, etc.) of the tackifying polymer to increase the compatibility with the crystalline polymer.
U.S. Pat. No. 5,412,035 (corresponding to JP-A-6-510548) discloses a pressure sensitive adhesive composition which becomes pressure sensitive at least at one temperature in the range between 20° C. and 40° C., and comprises a polymeric pressure-sensitive adhesive component and a crystalline polymer.
The crystalline polymer is usually non-tacky at room temperature and molecularly designed so that it has good compatibility with the polymeric pressure-sensitive adhesive component.
The melting point Ta (° C.) of the crystalline polymer, which is measured in the composition, is lower than the melting point Tm (° C.) of the crystalline polymer as such, and the difference Tm—Ta is preferably from 1° C. to 9° C. In the above U.S. patent, Tm is preferably from 20° C. to 102° C.
The adhesive composition disclosed has the thermal-peeling-easy properties (thermal peelability), that is, a peel strength P2 (g/cm) at a certain temperature higher than Ta is smaller than a peel strength P1 (g/cm) at a certain temperature lower than Ta. This publication discloses neither the use of crystalline polyols such as polycaprolactone nor the improvement of the molecular structure (e.g. the kinds of the functional groups and/or substituents in the molecule, etc.) of the tackifying polymer to improve the compatibility with polycaprolactone and make it compatible with polycaprolactone at a temperature higher than the melting point of polycaprolactone. Furthermore, this publication does not disclose the improvement to increase the thermal peelability and the adhesion properties at low temperature (low temperature adhesion properties) of such an adhesive composition at the same time.
JP-A-2000-119624 discloses an adhesive composition comprising a specific tackifying polymer and crystalline polycaprolactone. Using the disclosed adhesive composition, an article can be adhered to an adherent by heat pressing (pressing after heating or pressing while heating). It is essential for the disclosed tackifying polymer to have two functional groups, that is, a hydroxyl group and a phenyl group, in the molecule, and the compatibility with polycaprolactone is improved by the functions of these functional groups. With such an adhesive, the tack on the adhesion surface at room temperature (about 25° C.) can be avoided with the function of the crystalline polymer having the polyol units such as polycaprolactone in the molecule.
None of the above publications disclose thermally peelable adhesive compositions comprising crystalline polyurethane as a crystalline polymer having polyol units formed from a crystalline polyol as repeating units.